A valve operation control system for an internal combustion engine in which hydraulic pressure applied to a valve operation mode changing mechanism is controlled by hydraulic pressure control means formed from a holder mounted on a cylinder head, a spool valve formed by slidably housing in a valve body connected to the holder a spool valve body having one end part facing a pilot hydraulic chamber, and an electromagnetic open/close valve for controlling the hydraulic pressure of the pilot hydraulic chamber is already well known from, for example, Patent Publication 1, etc.    Patent Publication 1: Japanese Patent Application Laid-open No. 2002-155718